A variety of different metal and polymeric composites are known and are used in many, varied applications. One important use for a metal/polymer composite is as a shield for a variety of electromagnetic and radio frequency waves. Such waves, which cause interference in a variety of devices, are commonly referred to as electromagnetic interference (EMI) and radio frequency interference (RFI) (hereinafter jointly referred to as EMI). Shielding from EMI is often placed around an EMI source to prevent it from radiating EMI and interferring with surrounding devices. Also, the devices themselves may be provided with EMI shielding in an effort to shield the device from incoming EMI.
Another important use for a metal/polymer composite is for the protection of sensitive electronic parts from static charges. Static charge build-up can result from, for example, friction between surfaces, and can lead to a build-up of a high electrical potential. A sensitive electronic part that may come into proximity or contact with a statically charged surface can be destroyed or damaged. During shipping or handling, shielding of an electronic part from static electricity can be accomplished by placing the part in an electrically conductive metal or metal/polymer container, with the metal providing a preemptive surface to drain away any static charge.
Metal is a well-known, effective shield against EMI. However, many applications require that the shielding be transparent, while also being electrically conductive to aid in the dissipation of EMI. In addition, many applications require that the EMI shielding be thermoformed into a required shape or structure. A thermoforming process comprises heating the material and forming the material into the desired shape. As used herein, thermoforming refers to both stretching a material while heated and also heat-shrinking a material. Metal is also relatively heavy compared to polymeric materials, and therefore, where weight is a factor the amount of metal is desirably reduced.
Another desirable characteristic of an EMI or static shielding is of course cost. Since many metals are relatively expensive, it is desirable to be able to utilize sufficient metal to accomplish the objective, while closely tailoring the amount of metal to that desired amount. This also allows the weight of the composite to be kept to a minimum.
In addition to the cost of the metal itself, metal fabrication is generally a costly procedure. Separately fabricating a wire mesh or screen for lamination to a polymer is relatively costly. Further, handling of wire meshes can present difficulties, especially for fine meshes or screens of soft metals.
Metal/polymer composites having continuous coatings of metal are known. Continuous metal coatings are typically deposited by vapor deposition, sputtering or the like. To obtain coatings thick enough for good electrical conductivity, such processes are typically time consuming and relatively expensive. Also, such coatings are not transparent, and are therefor undesirable for certain uses.
Therefore, a need exists for an economical metal/polymer composite which may be used, for example, in EMI or static shielding. There is also a need for a metal/polymer composite which is electrically conductive, transparent, and which, if desired, is able to be thermoformed, without loss of either electrical conductivity or transparency. There is also a need for such a metal/polymer composite which includes precisely arranged fine metal strands, the amount of metal closely tailored to that amount necessary to accomplish the desired task. There is a further need for a metal/polymer composite with a metal layer comprising a plurality of fine metal strands in a random configuration. Further, there is a need for a method of making a suitable metal/polymer composite without the need for separate fabrication of a metal structure, such as a wire mesh or screen.